(1) Field of the Invention
The present invention relates to a bonded can excellent in the resistance to hot water and also to an undercoating composition for use in manufacture of this bonded can. More particularly, the invention relates to a bonded can in which a portion of a can body bonded by an adhesive is prevented from deterioration even under such conditions that the bonded portion falls in contact with hot water or steam, and also to an undercoating composition for use in manufacture of such bonded can.
(2) Description of the Prior Art
In the field of manufacture of cans, so-called tin-free steels (TFS) such as chromium-plated steel plates and chromic acid-treated steel plates having a chromium oxide film on the surface have been broadly used instead of tin-plated steel plates as metal materials for production of cans. Since soldering of these tin-free metal materials is very difficult, a side seam of a can body is mainly formed according to a method comprising bonding both the confronting side edges of a coated metal material for a can body to each other by an organic adhesive such as a polyamide. According to this known bond-seaming method, as disclosed in the specification of U.S. Pat. No. 3,663,354 to Ueno et al., an undercoating composition such as an epoxy-phenolic resin is first applied to both the surface of a metal material for a can body, melting a polyamide type adhesive located between both the confronting side edges of the metal material, and cooling and solidifying the melt under application of a pressure to bond both the side edges and form a can body. In short, according to this known method, in the bonded portion of the can body, the side edges are bonded to each other by the polyamide adhesive through the undercoat of an epoxy-phenolic resin coated on the surfaces of the metal material.
The bonded portion of a known bonded can prepared according to such known method has a satisfactory bonding strength under normal conditions and can sufficiently resist the pressure of a content having a spontaneous pressure such as a carbonated drink. However, the bonded portion of this known can is still insufficient in the resistance to hot water. In case of carbonated drinks, filling of a content into a can body is ordinarily carried out in a cold state, and since the content per se is acidic, a severe sterilization treatment need not ordinarily be performed for preservation of the content. Therefore, when a carbonated drink or the like is filled, the hot water resistance of the bonded portion is not particularly significant. However, in case of other various foods and drinks, for example, fruit juices and processed foods, from the viewpoint of preservation of contents, it is necessary to perform a severe heat sterilization treatment or a retort sterilization treatment or to carry out filling of contents in a hot state, and therefore, the bonded portion is required to have a high resistance to hot water. More specifically, if the bonded portion is poor in the resistance to hot water, the can body is readily broken at the heat sterilization or air tightness is lost in the content during preservation. This tendency is conspicuous when the interior of the can is kept in vacuum or under a reduced pressure.
On the other hand, bonded cans are advantageous in various points. For example, the kind of the metal material to be used is not particularly critical, and the can manufacturing speed, namely the speed of formation of side seams, is very high and the productivity is therefore very high. Accordingly, if a can body provided with a side seam formed by an adhesive and having a high hot water resistance be obtained, it is apparent that various advantages will be attained as regards the manufacturing cost and the problem of the resource.